The present invention relates to a centrifugal separator for separation of solids from a liquid mixture, in which particles of this kind are suspended in a liquid having a density smaller than that of the particles. Particularly, the invention concerns a so-called decanter centrifuge, which includes a rotor having a center axis around which it is rotatable at a first speed, a screw conveyor, which is arranged in the rotor and is rotatable around said center axis at a second speed differing from said first speed, and a driving device adapted for rotation of the rotor at said first speed and the screw conveyor at said second speed.
Decanter centrifuges of this kind are used in many different applications, especially where the mixture to be treated has a relatively large content of solids. Decanter centrifuges are used also in applications where the particles to be separated have a greatly varying size and/or where the content of particles in the mixture varies greatly. Furthermore, it is common to use decanter centrifuges in applications where relatively large volumes of liquid are treated, which means that decanter centrifuges as a rule are relatively large, so that they may give a satisfactory separation result despite large flows per unit of time of a supplied mixture.
Irrespective of the amount of mixture supplied per unit of time it is a desire for each centrifugal separator, independent of type, that it should provide a satisfactory separation result independently of whether the supplied liquid mixture has a relatively large or a relatively small content of solids. Thus, it is desirable that a centrifugal separator dimensioned for a certain through flow of liquid to be treated may be used under different conditions, i.e. both in conditions where the content of solids is large and under conditions where the content of solids is small. If the centrifugal separator has to be modified in order to give a satisfactory separation result in one condition or another, it should have a construction making such a modification possible in an inexpensive and simple way.
This is a problem with previously known decanter centrifuges, i.e. they have a construction which does not make a simple and inexpensive modification possible so the centrifuge becomes efficient, i.e. gives a satisfactory separation result, under different conditions.
It has become a consequence of this problem that, instead of decanter centrifuges, centrifugal separators of other kinds have been used under conditions where the mixture to be treated has had a relatively small flow per unit of time and an although relatively small but still not insignificant content of solids. Then, in certain cases, it has been possible to use so-called nozzle separators, but if the content of particles in the mixture, or the size of the particles, has varied heavily, nozzle separators have not been suitable. Instead, for this kind of separation cases, it has been necessary to use centrifugal separators, which are adapted for intermittent discharge of separated particles. However, other problems have then arisen, since centrifugal separators of this kind have a very limited ability to discharge separated solids during operation. Thus, it has either been necessary to perform sludge discharge operations at a very large frequency during the operation of the centrifugal separator, which has made an effective separation difficult, or it has been necessary to reduce the amount of mixture supplied to the centrifugal separator per unit of time, which has rendered the capacity of the centrifugal separator to be much too small. In both cases it has often been necessary to overdimension the centrifugal separator to obtain an acceptable separation efficiency, i.e. a satisfactory separation result, or an acceptable separation capacity.
In other connections, where a liquid to be treated has had a relatively large flow per unit of time and a relatively small content of solids, decanter centrifuges have been used, since use of centrifugal separators of a different kind has not been possible from a cost point of view. In these cases it has been necessary, however, to overdimension the decanter centrifuges so that they would give a desired separation result. Thus, it has not been possible to design a decanter centrifuge in an optimum way for treatment of a mixture having a relatively small content of solids.
Based on the foregoing, it is the general object of the present invention to provide a design or construction for decanter centrifuges which overcomes the problems and drawbacks of the prior art.
A primary object of the present invention is to provide a design or construction for decanter centrifuges, which is of a kind such that a decanter centrifuge relatively simply and inexpensively may be adapted for operation in connections where the mixture to be treated has a large content of solids as well as in connections where this content is small, so that a desired separation result is achieved.
Another object of the invention is that the design in question should make possible production of decanter centrifuges at a low cost.
The above defined objects may be obtained according to the invention by a design for a centrifugal separator of the initially defined kind, which is characterised by the combination that
the rotor is rotatably supported only at its one end through a rotor shaft, which is arranged such that the center axis of the rotor extends substantially vertically,
the rotor has an inlet for said mixture in the form of at least one inlet channel, which extends into the rotor at its said one end, a liquid outlet for separated liquid in the form of at least one outlet channel, which extends out of the rotor at its said one end, and a sludge outlet for separated solids situated at the opposite other end of the rotor,
the rotor includes a conical portion, at the apex of which the said sludge outlet is situated,
the screw conveyor is formed for transportation of the separated solids through said conical portion of the rotor towards the sludge outlet, and
the screw conveyor has or is connected with a conveyor shaft, which extends axially through the rotor shaft and is coupled to said driving device.
Thanks to this combination according to the invention said sludge outlet may be positioned at any desired distance from the rotor center axis all the way into this center axis.
A first advantage of the present invention is that it makes possible a dimensioning of a decanter centrifuge outgoing from the amount of solids to be discharged through the sludge outlet per unit of time. This means that a decanter centrifuge according to the invention, which is intended for a certain flow per unit of time of the mixture to be treated, may be given a desired separation ability without being overdimensioned, independently of the content of solids of the mixture.
A second advantage of the invention is that the sludge outlet of the rotor may be moved by simple means towards or away from the rotor center axis for adaptation of the decanter centrifuge to different needs or in connection with investigations for determination of the most suitable location of the sludge outlet in a special separation case.
A third advantage of the invention is that is suitable for relatively small decanter centrifuges, i.e. decanter centrifuges which may be used for treatment of liquid mixtures having a relatively small flow per unit of time. In small decanter centrifuges of this kind the sludge outlet may be formed as a very small opening at the apex of the conical rotor portion, i.e. at the innermost part of it at the rotor center axis, whereby the surrounding wall of the rotor may be given the smallest possible diameter. Hereby, the invention may be used in flow areas where conventional decanter centrifuges have not previously been used.
Small decanter centrifuges designed according to the invention may be produced very inexpensively, because many parts thereof may be produced for instance of plastics or light metal. A consequence thereof is that it may sometimes be suitable to use several relatively small decanter centrifuges designed according to the invention giving a desired separation result, instead of one or a few large conventional decanter centrifuges, which despite their size would still not give a desired separation result or which due to overdimensioning would be unnecessarily expensive to produce.
Preferably, said conical portion of the rotor has the shape of a hollow truncated cone, which at its narrow end forms an axially directed central opening, the sludge outlet of the rotor being formed by this opening. Upon change of the radial position of the sludge outlet either the hollow truncated cone may be exchanged in its entirety or a piece may be applied or removed from the narrow end of the hollow cone. If desired, the screw conveyor may be made so long that it extends out through the central opening, if the sludge outlet is to be situated relatively far from the rotor center axis. Alternatively, even the end portion of the screw conveyor may be exchangeable for screw pieces of different lengths.
Since the rotor is rotatably supported only at its upper end, i.e. the supporting device for the rotor has no bearing at a certain level in the area of the lower end of the rotor, the rotor, if desired, may be provided with a different conical portion, which is longer or shorter than the original conical portion, and a different screw conveyor adapted thereto. This may be desirable, for instance, if the consistency of the sludge to be separated in the rotor would require in a certain connection a more or less steep inclination relative to the rotational axis of the rotor when transported by means of the screw conveyor towards the sludge outlet in the conical portion of the rotor.
As said previously, the design according to the invention is of a kind such that the sludge outlet of the rotor may be placed very close to the rotational axis of the rotor. This also means that the free liquid surface, which is formed within the rotor during operation, may be kept relatively close to the rotational axis of the rotor, and this in turn makes possible that a separation efficiency improving insert of separation discs, e.g. conical separation discs, may be arranged within the rotor and have relatively small radial dimensions.
The possibility given by the invention for an arrangement of the sludge outlet very close to the rotational axis of the rotor may alternatively be used in a way such that separated sludge is given an extra long way of transportation along a liquid-free part of the conical portion of the rotor. In certain separation cases this may be desirable for the achievement of a sludge as dry as possible. In a preferred embodiment of the invention the rotor shaft and the conveyor shaft are coupled together through a gear device, which includes three co-operating gear members, of which a first gear member is connected with the rotor shaft and a second gear member is connected with the conveyor shaft, said three gear members being adapted for rotation relative to each other around a prolongation of the rotor centre axis and said inlet channel extending centrally through the gear device.
This gear device may be a planetary gear device, but preferably it is constituted by a so-called Harmonic Drive gear device (HD gear device) including a stiff cylindrical gear member, which is rotatable around its center axis and has a first number of cogs or teeth distributed around this central axis, a flexible gear member, which extends around the same center axis and has a different number of cogs or teeth, which are distributed around the center axis and which are adapted gradually to be brought into and out of engagement with the cogs or teeth of the cylindrical gear member, and a wave generator which is adapted gradually to deform the flexible gear member and, thereby, accomplish said cog engagement between the gear members. Upon use of an HD gear device a very compact gear device can be obtained despite the previously mentioned inlet channel extending centrally therethrough. An HD gear device has previously been suggested for use in a decanter centrifuge (see U.S. Pat. No. 3,419,211 and U.S. Pat. No. 3,482,770). However, in that connection no inlet channel for mixture to be treated in the decanter centrifuge has extended centrally through the HD gear device.
The advantages of the design according to the invention may be used to a maximum if the screw conveyor and the rotor are not provided with any particular bearing device in the area of the sludge outlet. This is possible if the screw conveyor is journalled through its conveyor shaft at two axially spaced places in the rotor shaft, through which the conveyor shaft extends. It is also possible if the screw conveyor is allowed to abut by its conveyor flights against the inside of the rotor, e.g. in the conical portion of the rotor. If the screw conveyor and/or the rotor are made of plastics, an abutment of this kind may serve as journalling for the screw conveyor, at least in connection with starting of the rotation of the rotor and the screw conveyor. During normal operation, when the screw conveyor is loaded axially as a consequence of its transportation of sludge relative to the rotor, a certain small radial play may be allowed to come up between the rotor and the screw conveyor.
It is alternatively possible to use the technique which can be seen from U.S. Pat. No. 4,828,541 and according to which the screw conveyor is journalled relative to the rotor only at its one end and for the rest is formed in a way such that it may float on the liquid present in the rotor during operation. If the screw conveyor, as mentioned earlier, is made of plastics, it will in many cases, just as a consequence thereof, be floating on the liquid present in the rotor during operation.
A Vertical arrangement for decanter centrifuges has previously been suggested, for instance in U.S. Pat. No. 2,862,658 and U.S. Pat. No. 5,364,335. However, each one of the decanter centrifuges disclosed in these patents does not have the whole combination of different design features, which constitutes the present invention. Thus, in the decanter centrifuge according to U.S. Pat. No. 2,862,658 part of the rotor sludge outlet opening in the conical rotor portion is occupied by two stationary pipes; one inlet pipe for mixture to be treated in the rotor and one outlet pipe for a separated liquid fraction. These two pipes make impossible a form of both the rotor and the screw conveyor such that the sludge outlet can be placed very close to the rotor center axis. Also at the decanter centrifuge according to U.S. Pat. No. 5,364,335 it is impossible to locate the sludge outlet very close to the rotor center axis, since in this case the rotor and the screw conveyor are interconnected through a gear box at the apex of the conical portion of the rotor. Thus, none of these known decanter centrifuges, having a vertical rotational axis, has a design fulfilling the previously mentioned primary object of the present invention.
Within the scope of the invention it is also possible to use the technique described in U.S. Pat. No. 3,795,361 and U.S. Pat. No. 3,934,792. In accordance with this technique the screw conveyor is provided with a flange or a partition, which divides the interior of the rotor into two chambers; one separation chamber closest to the liquid outlet and one sludge outlet chamber closest to the sludge outlet. Said partition leaves closest to the surrounding wall of the rotor a narrow slot, which connects the separation chamber with the sludge outlet chamber. By proper setting of the relative speed between the rotational movements of the screw conveyor and the rotor, respectively, separated sludge may be transported through this slot during the operation of the centrifugal separator at a speed such that the slot is constantly kept blocked by sludge. Thereby, the sludge prevents a free flow of unseparated liquid from the separation chamber into the sludge outlet chamber.
If desired, a free liquid surface may be maintained in the separation chamber at a level radially very close to or even radially inside the level of the sludge outlet. Hereby, separated sludge in the radially outermost part of the separation chamber may be subjected to an increased hydraulic pressure from the liquid in the separation chamber, which may act compressing on the sludge. Simultaneously, a hydraulic force is obtained from the liquid in the separation chamber, which contributes to the passage of the sludge through the aforementioned slot from the separation chamber to the sludge outlet chamber.
Depending upon the consistency of the sludge entering the sludge outlet chamber this chamber will contain a larger or smaller amount of sludge during the operation of the decanter centrifuge. If the sludge is relatively dry, the screw conveyor may displace it gradually towards and out through the sludge outlet. If the sludge is relatively wet or contains parts more liquid than solid, the whole of the sludge outlet chamber may be filled with sludge. If so, the screw conveyor may transport relatively solid parts of the sludge closest to the surrounding wall of the rotor, whereas liquid or semi-liquid parts of the sludge will run out through the sludge outlet.
A further advantage can be achieved by use of a partition of the kind described above as a consequence of the fact, as mentioned, that a free liquid surface can be maintained in the separation chamber radially inside the sludge outlet. This makes it possible, namely, that separation discs, e.g. a set of conical separation discs, may be arranged very centrally in the centrifugal rotor. Separation discs of this kind thereby may be made relatively small and they will then become inexpensive to produce. The separation discs may be mounted for rotation either together with the rotor or together with the screw conveyor.
Separation discs, e.g. conical separation discs, are desirable particularly when the mixture to be treated contains small and only with difficulty separable particles. Upon treatment of a mixture of this type it is in addition often difficult to obtain a separated sludge having a large dry substance content. The use of a partition of the above described kind as well as a set of separation discs, e.g. conical separation discs, may give a combination effect for the achievement of a desired separation result in separation cases like this.
A partition of the said kind may be formed and arranged in different ways. For instance, it may be formed as a plane annular disc, which is connected with the screw conveyor and is arranged coaxially therewith. It may be placed in the conical portion of the rotor or, if the rotor also has a cylindrical portion, preferably in the area where the conical portion is connected with the cylindrical portion.
Alternatively, the partition may extend substantially in an axial plane, in which also the rotational axis of the screw conveyor extends, and bridge the gap between two axially adjacent parts of one conveyor flight of the screw conveyor. In a case like this, part of the conveyor flight itself forms part of the partition separating the separation chamber of the rotor from the sludge outlet chamber.